1. Field of the Invention
The invention presented herein relates to liquid level sensing systems utilizing sensing electrodes positioned at various levels for contacting the liquid to connect the liquid as a part of electronic circuitry which provides electrical signals indicative of the sensed liquid level.
2. Brief Description of the Prior Art
U.S. Pat. No. 3,911,744 discloses a liquid level sensing arrangement wherein single sensing electrodes spaced a short distance from a common electrode are positioned at a multiplicity of levels at which it is desired that liquid levels be detected. The sensing electrodes and the common electrode are positioned within an open-ended probe unit that is inserted into the liquid storage container. A single multiconductor sheathed cable is used to connect each electrode to a separate fixed contact of a multiple position selector switch having a movable contact that is connected to a voltage source. Each fixed contact has a resistor which connects the contact to one input of a circuit for driving a light emitting diode. The other input for the circuit is connected to the common electrode for the sensing electrodes. When the liquid is present at a sensing electrode to which the selector switch is selectively connected, the light emitting diode is energized. The circuit for driving the light emitting diode utilizes an analog amplifier as an active input component which is also used to drive a meter used to provide an accurate indication of the liquid level.
Such prior art liquid sensing arrangement is not suitable for use in measuring levels for liquids containing materials which may cause a buildup of sediment at the electrodes. Sediment buildup may be a problem in clothes washers, for example. In addition, the connections that must be made to the separate sensing electrodes and corresponding switch connectors by the use of a multiconductor sheathed cable is not cost effective with respect to materials and the level of care and skill required to make proper connections. Further, positioning of the various electrodes as shown in the prior patents is unduly complicated.
Microprocessors are currently being utilized for controlling the operation of various types of apparatus making it desirable that a liquid level sensing system provide signals that are directly compatible with microprocessor requirements. In the case of the above-mentioned prior art system, the initial active input component is an analog device and, therefore, provides an output which is not directly compatible with microprocessor requirements.